Hearth Sense HearthSense A-Series Natural Gas Vent-Free Insert-20,000 BTU, Millivolt Control Fireplace Insert, Medium, Brown 20,000 Maximum BTU using Natural Gas Heats up to 1,000 sq. ft. Operates off Natural Gas Only Millivolt control valve, Manual Control and Remote Capable Hi/Low Adjustment - 14,000 BTU on Low Setting Zero Clearance Insert allows for custom built-in applications Realistic concrete logs and burner design Vent free gas fireplace insert Decorative arch style that fireplace perfect for any decor Economical: 99.9% efficient. No outside duct or chimney needed!